Sounds stupid to me. My brother and I both had 273's most of our lives growing up. My 4 speed Barracuda was never shifted less than 3,000 rpm and was usually wound up to 6,000 for fun. Redline was 5,200 for the Commando and that really small cam. My brother had a 67 auto Barracuda, 273 2 barrel, converted to 4 barrel with just the intake, carb and quick curve distributor. He had a vacuum gage but he never allowed the trans to shift on it's own. He shifted by sound and feel. I'm sure that engine saw rpm well above 4,500 on more than a few occasions. It is still running with 300K on the odometer.